Process for etching copper base materials

ABSTRACT

A process for etching copper or a copper base material with an aqueous solution consisting essentially of peroxydisulfuric acid, up to about 500 ppm of a halide addition and the balance water is described. The process includes off-line generation/regeneration of the etching solution. An electrochemical cell for generating/regenerating the etching solution is also described.

This application is a continuation of Ser. No. 410,000 filed Sept. 19,1989, which in turn was a continuation of Ser. No. 715,317 filed Mar.25, 1985, which in turn was a continuation-in-part of Ser. No. 539,598filed Oct. 6, 1983, all now abandoned.

This invention relates to a process and apparatus for etching copper andcopper base alloys using an aqueous peroxydisulfuric acid solution.

The fundamental cleaning medium or etchant used in the copper industryis sulfuric acid at a strength of about 10 to 15% by volume. Theextensive use of sulfuric acid is based on the fact that for mostbrasses and high copper alloys, the annealing oxides are readily solublein mineral acids. This produces very rapid oxide removal and theresulting cleaning process is, therefore, readily amenable to a shortimmersion time, continuous operation.

However, an increasing number of copper alloys now contain elementswhich form insoluble, refractory-type oxides. For these alloys,alternative cleaning or etching procedures have been developed. Thesealternative procedures generally involve adding an oxidant such assodium dichromate or hydrogen peroxide to the cleaning solution. Whilesodium dichromate has been used effectively for years, its use has beendiscontinued because of its known high toxicity and restrictions onwaste treatment and discharge level. While hydrogen peroxide does notsuffer from these disadvantages, it is limited by its inherent stabilityproblems. Hydrogen peroxide will decompose to water plus oxygen duringstorage. In addition and more importantly, the decomposition of peroxideis greatly accelerated by elevated temperatures and/or the presence ofdissolved copper in the cleaning solution. These factors limit themaximum etching rates that can be obtained with solutions containinghydrogen peroxide.

In response to these shortcomings and the need for an effectiveoxidizing cleaning solution, attention has turned to persulfatesolutions. The use of persulfate is attractive for copper cleaningbecause the end products of its reaction with copper are sulfuric acidand copper sulfate. As such, no special waste water treatment isrequired. Furthermore, a depleted or spent persulfate solution may beregenerated and the copper in the depleted solution may be reclaimed.Typically, persulfate cleaning solutions comprise an aqueous solutioncontaining a persulfate salt such as ammonium persulfate or sodiumpersulfate. Persulfate salts are used because they are readily availableand more stable than other forms of persulfate such as peroxydisulfuricacid. U.S. Pat. No. 2,978,301 to Margulies et al., U. S. Pat. No.3,475,242 to Radimer, U.S. Pat. No. 3,671,344 to Chiang et al., U.S.Pat. No. 3,887,405 to Fong et al. and U.S. Pat. No. 4,144,144 to Radimeret al. illustrate the use of aqueous persulfate salt solutions to etchcopper materials.

The use of electrochemical cells to regenerate spent solutions and toreclaim metal values from the spent solutions is also well known in theart. Typically, these cells have at least one anode chamber and at leastone cathode chamber physically separated by a membrane. Depending uponthe type of solution being regenerated and the chemical reactions soughtto be produced, the membrane may comprise either an ion exchange memberor a microporous separator. Depending upon the process, the spentsolution may be used as either the anolyte or the catholyte in the cell.U.S. Pat. No. 2,748,071 to Eisler, U.S. Pat. No. 2,865,823 to Harris etal., U.S. Pat. No. 3,761,369 to Tirrell, U.S. Pat. No. 3,764,503 toLancy et al., U.S. Pat. No. 4,051,001 to Inoue et al., and U.S. Pat. No.4,149,946 to Burke illustrate some of the electrochemical cells forregenerating spent solutions.

It is known in the prior art to regenerate persulfate etchants usingelectrochemical cells. In one such regeneration technique, the spentpersulfate etchant is first treated to remove a mixture containingammonium persulfate and the corresponding metal sulfate. This mixture isthen transferred to the cathode chamber of the electrochemical cellwhere it is used as the catholyte. The remainder of the spent solutionis transferred to the anode chamber of the cell where it is used as theanolyte. The cathode and anode chambers of the cell are separated by adiaphragm which permits at least hydrogen ions to pass freely betweenthe anolyte and the catholyte while preventing any substantial amount ofpersulfate in the anolyte from mixing with the catholyte. By passing anelectric current between the anolyte and catholyte, metal is plated outat the cathode and persulfate values are produced at the anode. U.S.Pat. No. 3,406,108 to Radimer et al. illustrates this technique forregenerating spent persulfate etchants. The primary deficiency of thistechnique is its complexity which renders it commercially unacceptable.

A second and simpler technique is illustrated in U.S. Pat. No. 3,470,044to Radimer. In this technique, the spent aqueous ammonium persulfateetching solution is used as the anolyte in an electrochemical cell. Anelectrolyte such as an acidic bisulfate or a bisulfate containingelectrolyte is used as the catholyte of the cell. The cathode and anodesections of the cell are separated by a cationic exchange membrane whichpermits the dissolved metal ions to pass from the anolyte into thecatholyte but which prevents any substantial amount of persulfate in theanolyte from mixing with the catholyte. By passing an electric currentthrough the catholyte and the anolyte, dissolved metal is removed fromthe solution at the cathode and sulfate values are converted topersulfate values at the anode.

While simpler, this second technique is believed to be inefficient andcommercially unacceptable. The production of temperature sensitive,oxidizing cleaning solutions such as persulfate etchants often requireelectrolyte temperatures to be maintained within certain critical limitsduring processing. Therefore, an electrochemical cell for regeneratingsuch a temperature sensitive solution should have some means forcontrolling electrolyte temperature. Furthermore, special anodes areoften required to improve cell efficiency. Such temperature controlmeans and special anodes are known in the art. U.S.S.R. Pat. Nos.311,502 and 470,307 both to Markov et al. illustrate some special anodeconstructions.

It is an object of the present invention to provide a process foretching copper and copper base materials with an aqueousperoxydisulfuric acid solution.

It is a further object of the present invention to provide a process andapparatus for electrolytically generating the peroxydisulfuric acidetching solution.

These and further objects will become apparent from the followingdescription and drawings in which like elements have been given likereference numbers.

In accordance with the present invention, a relatively simple but yethighly efficient process for etching copper and copper base materials isprovided. The process comprises preparing off-line an etching solutionconsisting essentially of peroxydisulfuric acid in an effective amountup to about 6N. and up to about 500 ppm of a halide addition,introducing the etching solution into a cleaning tank and immersing thecopper or copper base material in the etching solution. Within thecleaning tank, the peroxydisulfuric acid concentration of the etchingsolution is maintained in a range from about 0.2N. to about 5.5N.,preferably from about 0.5N. to about 2N. If needed, the concentratedetching solution may be diluted with water when introduced into thecleaning tank to obtain the desired working concentration ofperoxydisulfuric acid. During cleaning, a desired peroxydisulfuric acidconcentration may be maintained by bleeding concentratedperoxydisulfuric acid into the solution in the cleaning tank.

As the amount of copper dissolved in the cleaning solution increasesand/or the amount of peroxydisulfuric acid becomes significantlydepleted, the etching rate or cleaning power of the solution decreases.When the cleaning power or etching rate of the solution reaches a levelwhere it is no longer feasible to clean the copper or copper basematerial within commercial time constraints, the solution is withdrawnfrom the tank and replaced by a fresh cleaning solution. The withdrawnspent cleaning solution may be either discarded or recirculated to anelectrolytic cell for regeneration.

The apparatus described herein in connection with the accompanyingdrawings provides a means for practicing the process of the presentinvention.

FIG. 1 is a schematic representation of a system for cleaning metalstrip in accordance with the present invention.

FIG. 2 is a partial cross-sectional view of an electrochemical cell forgenerating/regenerating a peroxydisulfuric acid etching solution.

FIG. 3 is a graph illustrating the etching rate of peroxydisulfuric acidas a function of its composition.

FIG. 4 is a graph illustrating the stability of peroxydisulfuric acid asa function of time and temperature.

FIG. 5 is a graph illustrating the stability of peroxydisulfuric acidwith a halide addition as a function of time.

FIG. 6 is a graph illustrating the effect of temperature on cellefficiency and peroxydisulfuric acid generation.

FIG. 7 is a graph illustrating the effect of current density on thegeneration of peroxydisulfuric acid using a cell similar to that shownin FIG. 2.

The present invention is concerned with effectively cleaning or etchingcopper base materials including copper and copper alloys. The copperbase materials to be cleaned may have any form. For example, thematerials may be in strip form, tube form, rod form, wire form or shapedinto an article. Furthermore, the cleaning may be carried out eitherbatchwise or continuously.

Referring now to FIG. 1, a system for cleaning a copper base material isillustrated. The copper base material not shown to be cleaned is passedthrough a cleaning tank 10 containing an etching solution in accordancewith the present invention. The copper base material may be passedthrough the tank 10 in any suitable manner known in the art. Ifnecessary, the material may be subjected to a mechanical cleaning actionsuch as abrasion or another pretreatment, e.g. being passed through aboiling caustic solution, prior to entering the tank 10. The type ofpretreatment used, if any, will of course depend upon the material beingcleaned and the nature of the contaminants being removed.

In accordance with the present invention, the etching solution in thetank 10 comprises an aqueous peroxydisulfuric acid solution consistingessentially of peroxydisulfuric acid in a working concentration of fromabout 0.2N. to about 5.5N., preferably from about 0.5N. to about 2N. Forreasons to be explained more fully hereinafter, up to about 500 ppm of ahalide addition may also be present in the solution. Peroxydisulfuricacid in the above concentrations has been found to be an effectivecleaning agent. While similar cleaning effectiveness has been obtainedusing persulfate salt cleaning solutions such as aqueous sodiumpersulfate or ammonium persulfate solutions, it is preferred inaccordance with this invention to use a peroxyacid solution rather thana persulfate salt solution for the following reasons. The presence ofadditional ions such as Na⁺ or NH₄ ⁺ ions in a persulfate salt solutioncan lead to the production of undesirable products, e.g. ammonia.Furthermore, they complicate any regeneration process because theadditional ions, i.e. Na⁺ or NH₄ ⁺, as well as the useful persulfateions have to be replenished.

The etching solutions of the present invention are also characterized bythe absence of chromium or chromate ions, significant amounts of othermineral acids, organic materials such as diazines and purines, andintentional metal ion additions. The absence of chromium, chromate ionsand organic materials is important from both environmental and economicstandpoints. These materials can be toxic and expensive waste treatmentfacilities are generally needed to treat them prior to disposal. Theabsence of intentional metal ion additions and significant amounts ofother mineral acids is important in that additional values do not haveto be generated initially and/or replenished during any regenerationprocess.

Since peroxydisulfuric acid tends to decompose more rapidly at elevatedtemperatures, it has been found to be particularly advantageous to carryout the present etching process while the solution is at a temperaturein the range of about 15° C. to about 50° C., preferably from about 20°C. to about 35° C. If needed, the tank 10 may be provided with asuitable cooling system not shown for removing heat and maintaining thecleaning solution within the desired temperature ranges.

To further retard the decomposition rate of the peroxydisulfuric acid inthe etching solution, the cleaning system may be provided with suitablemeans such as heat exchanger 16 for providing cooled etching solution tothe cleaning tank 10. It is believed to be desirable to provide theetching solution to the tank 10 at a temperature in the range of about0° C. to about 15° C., preferably from about 0° C. to about 10° C.

An aqueous peroxydisulfuric acid solution tends to decomposesequentially as follows:

    H.sub.2 S.sub.2 O.sub.8 +H.sub.2 O→H.sub.2 SO.sub.5 +H.sub.2 SO.sub.4                                                  (1)

    H.sub.2 SO.sub.5 +H.sub.2 O→H.sub.2 O.sub.2 +H.sub.2 SO.sub.4 (2)

    2H.sub.2 O.sub.2 →2H.sub.2 O+O.sub.2                (3)

It has been surprisingly found that as the peroxydisulfuric acid H₂ S₂O₈ decomposes to a combination of peroxydisulfuric acid H₂ S₂ O₈+peroxymonosulfuric acid H₂ SO₅ that the etching rate of the solutiontends to increase. Thereafter, the etching rate remains substantiallyconstant until H₂ S₂ O₈ changes almost completely.

As the copper or copper base material passes through the solution,copper ions will dissolve into the solution and the following reactionstypically take place:

    H.sub.2 S.sub.2 O.sub.8 +Cu→Cu.sup.2+ +H.sub.2 SO.sub.4 +SO.sub.4.sup.=                                           (4)

    H.sub.2 SO.sub.5 +Cu→CuSO.sub.4 +H.sub.2 O          (5)

    H.sub.2 O.sub.2 +H.sub.2 SO.sub.4 +Cu→CuSO.sub.4 +2H.sub.2 O (6)

As a result, the depleted etching solution tends to contain some H₂ S₂O₈, H₂ SO₅, H₂ O₂, H₂ SO₄ and Cu²⁺.

If too much copper is present in the depleted solution, CuSO₄ crystalscould form. To prevent this from happening, it is desirable to withdrawthe depleted solution before any significant copper ion level isreached. To this end, the tank 10 may be provided with means not shownfor monitoring the bath condition. The monitoring means may comprise anysuitable detection means known in the art to monitor any desiredparameter such as bath temperature and/or copper concentration.

The cleaning tank 10 may also be provided with means not shown foragitating the etching solution to insure that fresh cleaning speciescontact the copper or copper base material at all times. The agitatingmeans may comprise any suitable stirring device known in the art such asone or more impellers not shown driven by a power source not shown.

Because of peroxydisulfuric acid's thermal instability problems, it isdesirable to produce the etching solution of the present invention onsite. Consequently, the cleaning system includes an electrochemical cell14 for generating a desired concentrated peroxydisulfuric acid solution.Referring now to FIG. 2, a generation cell is illustrated. The cell 14comprises a central anode chamber 12 containing an anode 48 and a pairof cathode chambers 20, each containing a cathode 50. A physicalseparator 22 separates the anode chamber 12 from each cathode chamber20. The physical separators 22 are primarily used to prevent mixing ofthe products in the anode and cathode chambers and may be mounted in thecell 14 in any suitable manner. The physical separators 22 may either bea diaphragm such as a microporous polyethylene diaphragm or an ionexchange membrane such as a cation exchange membrane, e.g. a Nafionmembrane. A diaphragm would be used where it is desired to have arestricted bulk flow from one chamber to another without any preferenceto the charge of the ions passing therethrough. An ion exchange membranewould be used where it is desired to substantially prevent any bulk flowwhile permitting only the flow of certain perferred ions across themembrane.

In the cell embodiment of FIG. 2, the anode 48 may comprise at least onehollow tube or conduit through which for reasons to be explainedhereinafter a heat exchange medium flows, at least one current collector52 bonded to the outer surface of each tube and at least oneelectrochemically active portion 54 also bonded to the outer surface ofeach tube. In a preferred construction, the anode 48 comprises agrid-like structure formed by a plurality of spaced-apart substantiallyparallel tubes, a plurality of spaced-apart substantially parallelcurrent collectors 52 arranged substantially transverse to the tubes,and a plurality of electrochemically active portions 54 located on eachtube substantially adjacent to and between the location where eachcurrent collector 52 is bonded to each tube.

If desired, the anode 48 may comprise at least one solid rod having adesired cross sectional shape in lieu of the hollow tube(s). Where sucha construction is used, one or more current collectors and one or moreelectrochemically active portions may be bonded to the outer surface ofthe solid rod(s). The electrochemically active portions 54 may cover allor only selected portions of the rod(s). Alternatively, the anode 48 maycomprise any desired combination of solid rod and hollow tubeconstructions.

Each tube or solid rod is formed from an electrically conductivematerial. While any suitable electrically conductive material inert tothe electrolyte may be used, it has been found to be desirable to formeach tube or solid rod from titanium, tantalum, niobium or alloysthereof.

The current collectors 52 preferably each comprise a pair ofspaced-apart strips of electrically conductive material. Each strip maybe contoured to form good electrical contact with each tube or solidrod. The strips may be bonded to the tubes or solid rods in any suitablemanner known in the art, e.g. spot welding. To promote a substantiallyuniform current distribution throughout the cell, the current collectors52 are substantially uniformly distributed over the length of each tubeor solid rod. The number, the spacing and the size of the currentcollectors 52 depend upon the range of current densities used in thecell and the operating electrode surface area. While any suitable metalor metal alloy having good electrical conductivity properties may beused to form the current collectors 52, titanium, tantalum, niobium oralloys thereof are preferred materials. If desired, each currentcollector 52 may be formed from a composite material having a coreformed from a high conductivity material such as aluminum, copper andalloys thereof and a clad surface formed from a metal selected from thegroup of titanium, tantalum, niobium and alloys thereof.

The electrochemically active portions 54 are preferably formed by metalmembers bonded to the tubes or solid rods. For example, a plurality ofmetal rings can be spot welded to or inlayed in the tubes or solid rods.The extent of the portions 54 and their surface area depend upon therange of current densities used during cell operation. If needed, theportions 54 could cover the total electrically conductive area exposedto the anolyte. While the portions 54 may be formed from any suitablemetal or metal alloy, it is preferred to form them from platinum or oneof its alloys because platinum forms oxides which slow down oxygenevolution at the anode and provides a desired overvoltage. Brightrecrystallized platinum is a particularly useful material from which theportions 54 may be formed.

The cathode 50 in each cathode chamber 20 may be formed from a metallicmesh type structure such as a copper or copper alloy screen. It isdesirable that the mesh structure have at least about 50% of its surfacearea open to promote good mass transport from the surrounding catholyteand better usage of the physical separators 22. Preferably, each cathodehas about 50% to about 70% of its surface area open.

The anode 48 and the cathodes 50 may be electrically connected to anyconventional power supply not shown known in the art. Preferably, theanode 48 is connected to the current source or power supply via one ormore copper busses not shown attached to the current collectors 52. Thistype of cell construction and its operation is further described in U.S.Pat. No. 4,482,440 to Kadija, which is hereby incorporated by reference.

To generate peroxydisulfuric acid using the cell 14, an aqueous sulfuricacid solution is introduced into the anode and cathode chambers as theanolyte and the catholyte. The aqueous sulfuric acid solution may besupplied to the cathode and anode chambers by a circulation loop 26. Thecirculation loop 26 may have a pump 28 for providing the desiredcirculation. Preferably, the catholyte flows through the cathodechambers 20 in the same direction that the anolyte flows through theanode chamber 12. Circulation of the sulfuric acid solution is desirablefrom an efficiency standpoint.

It has been found that to generate the desired concentration ofperoxydisulfuric acid for the etching solution, the sulfuric acidsolution should contain from about 5% to about 50% by volume sulfuricacid, preferably from about 20% to about 25% by volume sulfuric acid. Ithas been found that such concentrations assist in reducing the activityof water and, therefore, the evolution of oxygen at the anode.

Since the persulfate ion is relatively unstable in solution and itsconcentration tends to decay rapidly in time, it is desirable tomaintain the catholyte and anolyte within a temperature range of about0° C. to about 20° C. during the regeneration process. Preferably, thecatholyte and anolyte are maintained at a temperature in the range ofabout 0° C. to about 10° C. Control of heat build-up in the anolyte alsohelps to favor the production of persulfate values as opposed to theproduction of peroxide values.

To maintain the temperature of the anolyte and the catholyte within thedesired limits, a heat exchange medium is circulated through the anodechamber 12. The heat exchange medium may comprise any suitable heatexchange fluid known in the art such as a refrigerated aqueous glycolsolution. The heat exchange fluid may be circulated through the chamber12 by a heat exchange loop not shown. The heat exchange loop maycomprise a pump not shown for circulating the heat exchange fluid, aunit not shown for cooling and/or heating the heat exchange fluid, andthe hollow tube or tubes forming part of the anode 48. The heat exchangefluid preferably flows through the heat exchange loop in a manner thatenables the fluid in each tube to travel in a direction counter to theanolyte and catholyte flow direction.

Alternatively, the anolyte and catholyte may be maintained within thedesired temperature range by including in the circulation loop 26, ameans 30 for cooling and/or heating the sulfuric acid solution before itenters the anode and cathode chambers. The heating/cooling means 30 maycomprise any suitable heat exchanger known in the art. If thisalternative approach is used, the hollow tube anode construction may bereplaced by the aforementioned solid rod anode construction and the heatexchange loop may be omitted.

It also has been found that the rate of decomposition of the persulfatevalues may be decreased by making a halide addition to the anolyte. Thehalide addition also tends to increase overall cell efficiency.Generally, about 10 ppm to about 500 ppm of a suitable halide such as achloride or a fluoride is added to the anolyte. If a chloride is beingadded, the addition should preferably be in the range of about 100 ppmto about 200 ppm. If a fluoride is being added, the addition shouldpreferably be in the range of about 50 ppm to about 100 ppm. While theaddition may be made using any form of the halide, it is preferred tomake an acid addition such as hydrochloric acid or hydrofluoric acid.

In operation, a voltage sufficient to cause a current density in theactive area of the anode in the range of about 0.4 A/cm² to about 1.0A/cm², preferably from about 0.6 A/cm² to about 0.8 A/cm², is applied tothe cell. A voltage in the range of about 4 to about 6 volts isgenerally sufficient to produce the desired current densities. Uponapplication of the voltage, sulfate ions are oxidized to persulfate ionsat the anode. There also tends to be some production of hydrogen gas atthe cathode and oxygen gas at the anode; however, the production ofthese gases does not hamper the generation process. In fact, they createa bubble lifting effect that promotes both anolyte and catholytemovement.

To provide the cleaning tank 10 with an etching solution having thedesired peroxydisulfuric acid working concentration, it may be necessaryto prepare in the cell 14 a concentrated solution having aperoxydisulfuric acid concentration as high as about 6N. While theminimum amount of peroxydisulfuric acid in the concentrated solution maybe as low as that needed to effectively etch copper or a copper basealloy, the preferred concentration is in the range of about 2N. to about5N. Most preferably, the peroxydisulfuric acid concentration is in therange of about 3.5N. to about 5N. When producing solutions have thehigher peroxydisulfuric acid concentrations, it is desirable to keep theanolyte and catholyte temperatures as low as possible within theaforesaid temperature ranges.

After the concentrated etching solution has been prepared, it may bedelivered to either the cleaning tank 10 or a storage tank 18. Thecleaning system preferably includes suitable ducting and valve(s) fordelivering the etching solution to either tank 10 or 18. As previouslydiscussed, the etching solution may be passed through a heat exchanger16 prior to being delivered to the tank 10. If the etching solution 10is to be stored in tank 18, the tank 18 may be provided with suitablemeans such as a cooling loop not shown for maintaining the solutiontemperature within the range of about 0° C. to about 15° C., preferablyfrom about 0° C. to about 10° C.

Prior to etching, the concentrated aqueous peroxydisulfuric acidsolution may have to be diluted to obtain a desired workingperoxydisulfuric acid concentration. To this end, tank 10 may beprovided with a water inlet 32. During etching, it may be necessary tobleed concentrated peroxydisulfuric acid into cleaning tank 10 tomaintain the desired working concentration of peroxydisulfuric acid. Theconcentrated peroxydisulfuric acid bleed solution may come from eitherthe storage tank 18 or the cell 14. If desired, concentrated aqueousperoxydisulfuric acid solution may be continually fed into the tank 10from either tank 18 or cell 14.

After the cleaning power of the solution has been depleted so thatetching cannot be carried out within a commercially acceptable time, thespent peroxydisulfuric acid solution may be withdrawn from the tank 10and either discarded or sent back to cell 14 to be regenerated. Aspreviously discussed, one of the advantages of the present invention isthat no special waste treatments have to be performed on the etchingsolution prior to it being discarded. The withdrawn solution may bereplaced by fresh solution from either cell 14 or storage tank 18.

If the withdrawn solution is to be regenerated, it is circulated back tothe cell 14 where it is introduced into the cell as the anolyte in lieuof the sulfuric acid anolyte. Regeneration may be carried out under thesame conditions of temperature, current density, voltage, concentrationof sulfuric acid in the catholyte, and halide addition as previouslyoutlined with regard to solution generation. The only significantdifference between solution generation and solution regeneration is thatthe withdrawn solution contains copper values which have to be removedduring the regeneration process. Copper removal may be effected byplating the copper values onto the cathode(s) and providing means forremoving the copper from the cathode(s). If the solution is beingregenerated in a batchwise manner, the entire cathode structure may beremoved and replaced. If the solution is being continuously regenerated,the cathode(s) may be formed from an endless metallic mesh belt notshown rotated by a suitable drive motor not shown. A metal removingdevice not shown such as a conventional scraping knife or blade may beused to remove the plated metal from the belt.

If desired, the withdrawn solution may be passed through a suitablecooling device such as heat exchanger 24 prior to being introduced intothe cell 14. In this way, the spent solution may be provided to the cellat a desired temperature. If adjustments in raw material concentrationare needed prior to solution regeneration, the withdrawn solution may befirst transferred from the cleaning tank 10 to a reconditioning unit notshown. The reconditioning unit may comprise any suitable solutionreconditioning means known in the art. Of course, the type ofreconditioning unit utilized will depend upon the type of adjustmentthat needs to be made to the spent solution. For example, thereconditioning unit may comprise means for adding one or more rawmaterials to the solution.

The following examples are presented to demonstrate the presentinvention.

EXAMPLE I

To compare cleaning power or etching rate, the following cleaningsolutions were prepared: (1) an aqueous peroxydisulfuric acid solution(PDA) containing 1.85M peroxydisulfuric acid and 200 ppm Cl⁻ maintainedat 25° C.; (2) a 12% H₂ SO₄ -3% H₂ O₂ -distilled water solutionmaintained at 43° C.; (3) a 12% H₂ SO₄ -3% H₂ O₂ -tap water solutionmaintained at 43° C.; (4) an aqueous 2N. ferric sulfate solutionmaintained at 80° C.; and (5) an aqueous 2N. ferric sulfate solutioncontaining 0.3M Cu²⁺ maintained at 80° C. The chloride addition to theperoxydisulfuric acid solution was made using HCl. 1"×2" coupons ofcopper alloy C11000 were prepared and weighed. Several coupons were thenimmersed in each solution for 15 seconds. After being removed, thecoupons were rinsed in water and reweighed. The procedure was thenrepeated again with each coupon being reweighed a second time. Theetching rate of each solution is reported in Table I.

                  TABLE I                                                         ______________________________________                                                               Etching Rate                                           Solution               (mg/cm.sup.2 /sec)                                     ______________________________________                                        1.85M peroxydisulfuric acid                                                                          .081                                                   w/200 ppm Cl.sup.-                                                            12% H.sub.2 SO.sub.4 --3% H.sub.2 O.sub.2 --distilled H.sub.2 O                                      .070                                                   12% H.sub.2 SO.sub.4 --3% H.sub.2 O.sub.2 --tap H.sub.2 O                                            .011                                                   2N. ferric sulfate     .120                                                   2N. ferric sulfate + 0.3M Cu.sup.2+                                                                  .095                                                   ______________________________________                                    

EXAMPLE II

A 1.6M peroxydisulfuric acid solution H₂ S₂ O₈ was prepared andpermitted to decompose at room temperature (22° C.) toperoxymonosulfuric acid H₂ SO₅. At various intervals, the solution wastested to measure its composition, in particular, the concentration ofperoxydisulfuric acid and peroxymonosulfuric acid. During theseintervals, coupons of alloy C11000 were immersed in the solution as inExample I. Prior to the first immersion and after each immersion, thecoupons were weighed so that the solution's etching rate could bedetermined. As can be seen from FIG. 3, the etching rate increased asthe H₂ S₂ O₈ decomposed into H₂ SO₅ until a point after the solution hada concentration of about 1.2M H₂ SO₅ and about 0.4M H₂ S₂ O₈.Thereafter, the etching rate was slightly decreased. FIG. 3 alsoillustrates the etching rate of the 1.6M peroxydisulfuric acid as itdecomposes compared to the etching rate of other cleaning solutions.

EXAMPLE III

1"×2" coupons of copper alloys C68800 and C65400 were prepared. TheC68800 coupons were prepared from C68800 strip that had been bellannealed. The C65400 coupons were prepared from C65400 strip materialthat had been strip annealed.

The C68800 samples were first immersed in a boiling caustic solutioncontaining 1N. NaOH. The solution was at about 100° C. Different couponsamples were immersed for different time intervals. The time intervalswere 10 seconds, 20 seconds and 30 seconds. After being rinsed, some ofthe samples were immersed in an aqueous 12% H₂ SO₄ -3% H₂ O₂ solutionwhich was at about 43° C. The remainder of the coupons were immersed inan aqueous peroxydisulfuric acid solution containing 1.8Mperoxydisulfuric acid and about 200 ppm Cl⁻ which was at about 25° C.The chloride addition to the peroxydisulfuric acid solution was madeusing HCl. The immersion time of the respective samples in each cleaningsolution reflected its immersion time in the boiling caustic solution.For example, if the sample had been immersed in the caustic solution for10 seconds, it was immersed in the acid cleaning solution for 10seconds.

The C65400 samples were divided into two groups. The first group wasimmersed in an aqueous 12% H₂ SO₄ -3% H₂ O₂ solution at about 43° C. forthe following time intervals: 20 seconds; 30 seconds; and 40 seconds.The second group was immersed in an aqueous peroxydisulfuric acidsolution containing 1.8M peroxydisulfuric acid and about 100 ppm F⁻ atabout 25° C. The fluoride addition was made as HF. The C65400 sampleswere immersed in the peroxydisulfuric acid solution for the followingtime intervals: 15 seconds; 25 seconds; and 35 seconds. None of theC65400 samples were subjected to a caustic solution pretreatment.

All of the samples were rinsed in water and dried after immersion in theacid cleaning solution. The samples were then evaluated by dip solderingin a 60% Sn-40% Pb solder bath at about 238° C. A resin flux was appliedprior to the dip tests. All samples were immersed in the bath for fiveseconds and then slowly pulled out. The solderability of the dippedsamples was subjectively rated using the following system:

    ______________________________________                                        Class           Description                                                   ______________________________________                                        1               Uniform smooth coating                                        2               Uniform rough coating                                         2a              <5% dewetting                                                 3               <50% dewetting                                                                <10% pinholes                                                 4               >50% dewetting                                                                >10% pinholes                                                 5               No adhesion                                                   ______________________________________                                    

As can be seen from Table II, the samples of C68800 and C65400 cleanedwith the peroxydisulfuric acid solution exhibited better solderabilityratings - the lower the class, the better the rating. With respect tothe C654000 samples, those cleaned in the peroxydisulfuric acid solutionexhibited better solderability ratings with shorter immersion times.

                  TABLE II                                                        ______________________________________                                                                Immersion                                                                     Time      Solderability                               Alloy   Treatment       (sec)     Rating                                      ______________________________________                                        C68800  NaOH/H.sub.2 SO.sub.4 --H.sub.2 O.sub.2                                                       10        4                                           C68800  NaOH/H.sub.2 SO.sub.4 --H.sub.2 O.sub.2                                                       20        3-4                                         C68800  NaOH/H.sub.2 SO.sub.4 --H.sub.2 O.sub.2                                                       30        3-4                                         C68800  NaOH/H.sub.2 S.sub.2 O.sub.8                                                                  10        2                                                   w/200 ppm Cl.sup.-                                                    C68800  NaOH/H.sub.2 S.sub.2 O.sub.8                                                                  20        2                                                   w/200 ppm Cl.sup.-                                                    C68800  NaOH/H.sub.2 S.sub.2 O.sub.8                                                                  30        2                                                   w/200 ppm Cl.sup.-                                                    C65400  H.sub.2 SO.sub.4 --H.sub.2 O.sub.2                                                            20        4-5                                         C65400  H.sub.2 SO.sub.4 --H.sub.2 O.sub.2                                                            30        3-4                                         C65400  H.sub.2 SO.sub.4 --H.sub.2 O.sub.2                                                            40        2a                                          C65400  H.sub.2 S.sub.2 O.sub.8                                                                       15        3                                                   w/100 ppm F.sup.-                                                     C65400  H.sub.2 S.sub.2 O.sub.8                                                                       25        2                                                   w/100 ppm F.sup.-                                                     C65400  H.sub.2 S.sub.2 O.sub.8                                                                       35        2                                                   w/100 ppm F.sup.-                                                     ______________________________________                                    

EXAMPLE IV

To demonstrate the effect of temperature on the stability ofperoxydisulfuric acid solutions, solutions initially containing 1.5M H₂S₂ O₈ and about 2.5M H₂ S₂ O₈ were prepared. None of these solutionscontained a halide addition. A portion of the 2.5M H₂ S₂ O₈ solution waskept at 0° C. and allowed to decompose for several days. The 1.5M H₂ S₂O₈ solution were kept at room temperature (about 22° C.) and allowed todecompose for several days.

As can be seen from FIG. 4, the solution at 0° C. retained asubstantially constant total molarity. As used herein, the totalmolarity is the sum of the peroxydisulfuric acid molarity and theperoxymonosulfuric acid molarity. In comparison, the total molarity ofthe solutions at room temperature deteriorated rather rapidly.

EXAMPLE V

To illustrate the effect of a halide addition on decreasing thedecomposition rate of peroxydisulfuric acid, an aqueous 1.6M H₂ S₂ O₈solution was prepared. 100 ppm of F⁻ was added to the solution usinghydrofluoric acid. The solution with the halide addition was kept atroom temperature and permitted to decompose.

As can be seen from FIG. 5, the amount of H₂ S₂ O₈ in the solutiondecreased rather rapidly while the amount of H₂ SO₅ in the solutionincreased. As a result, the total molarity, the sum of H₂ S₂ O₈ and H₂SO₅ in solution, remained substantially constant. It can be seen fromFIG. 5 that when the concentration of H₂ O₂ starts to decrease, theconcentration of H₂ O₂ increases. This clearly demonstrates thedecomposition mechanism of peroxyacids discussed hereinbefore.

By comparing the total molarity curve of FIG. 5 with the total molaritycurve of the 1.5M solution in FIG. 4, it can be seen that the additionof a halide to the peroxydisulfuric acid solution decreases its rate ofdecomposition as represented by total molarity.

EXAMPLE VI

To illustrate the effect of a halide ion addition on the generation ofperoxydisulfuric acid in an electrochemical cell such as the one shownin FIG. 2, several aqueous 20 v/o H₂ SO₄ solutions were prepared. Thefollowing additions were made to various samples of the H₂ SO₄ solution:(a) 100 ppm Cl⁻ ; (b) 200 ppm Cl⁻ ; and (c) 100 ppm F⁻ . The chlorideadditions were made as hydrochloric acid and the fluoride addition wasmade as hydrofluoric acid.

The solutions with the halide additions were used as the anolyte in thecell. For comparison purposes, an aqueous 20 v/o H₂ SO₄ solution withoutany halide addition was also used as an anolyte in the cell. For alltests, the catholyte was an aqueous 20 v/o H₂ SO₄ solution. The anodeand cathode chambers of the cell were separated by a Nafion membrane andan aqueous glycol solution at about 0° C. was circulated through theanode chamber during each test. During the peroxydisulfuric acidgeneration process, the anolyte was maintained at a temperature in therange of 5° C. to 8° C. A current density of 0.4 A/cm² at a voltage of5.5 V was applied to the cell.

As can be seen from Table III, the halide addition to the anolyte tendedto improve cell efficiency. As used herein, cell efficiency may bedefined as the amount of charge theoretically needed to convert 100% ofthe sulfuric acid to H₂ S₂ O₈ divided by the actual charge used togenerate the peroxydisulfuric acid.

                  TABLE III                                                       ______________________________________                                        Halide Addition                                                                            Peroxydisulfuric                                                                           Cell Efficiency                                     (ppm)        Acid (molarity)                                                                            (%)                                                 ______________________________________                                        None         1.6          11                                                  100 Cl.sup.- 1.6          22                                                  200 Cl.sup.- 1.8          37                                                  100 F.sup.-  1.8          40                                                  ______________________________________                                    

EXAMPLE VII

To demonstrate the effect of temperature on cell efficiency and H₂ S₂ O₈generation, an aqueous 20 v/o H₂ SO₄ solution containing 100 ppm F⁻ wasprepared as in Example VI and introduced into the cell of FIG. 2 as theanolyte. As before, an aqueous 20 v/o H₂ SO₄ solution without any halidewas used as the catholyte, Nafion membrane separated the anode andcathode chambers, and an aqueous glycol solution was circulated throughthe anode chamber. A current density of 0.6 A/cm² was applied across thecell.

As can be seen from FIG. 6, cell efficiency decreased as anolytetemperature increased. Furthermore, the rate of peroxydisulfuric acidproduction tended to be higher for the lower temperatures.

EXAMPLE VIII

To demonstrate the effect of currenty density on cell efficiency andperoxydisulfuric acid generation, an aqueous sulfuric acid solutioncontaining 100 ppm F⁻ was prepared as in Example VI and introduced intothe cell of FIG. 2 as the anolyte. The other cell conditions were thesame as in Example VII except that the anolyte temperature wasmaintained at 11° C. and a current density of 0.4 A/cm² was appliedduring one test and a current density of 0.6 A/cm² was applied during asecond test. It can be seen from FIG. 7 that the higher the currentdensity, the better the cell efficiency and the rate of production ofperoxydisulfuric acid.

While a particular type of electrochemical cell has been illustrated togenerate/regenerate the peroxydisulfuric acid cleaning solution, othertypes of electrochemical cells could be utilized.

While any suitable heat exchange fluid at any desired temperature may becirculated through the anode chamber to control the temperature of theanolyte as shown in FIG. 2, it is preferred to use an aqueous glycolsolution at a temperature of about -5° C. to about 5° C., mostpreferably at about 0° C., as the heat exchange fluid.

While the examples illustrate the use of peroxydisulfuric acid solutionsto clean copper alloys C11000, C65400 and C68800, the solution may beused to clean copper and other copper alloys. Depending upon the type ofcopper or copper base alloy forming the material to be cleaned and thenature of the contaminants on the material, it may be desirable topretreat the material prior to its immersion in the peroxydisulfuricacid solution. Any suitable pretreatment such as immersion in a boilingcaustic solution or mechanical action may be used.

While it is desirable to maintain the catholyte and anolyte temperatureswithin the generation/regeneration cell within the aforesaid temperatureranges, it is possible to generate/regenerate peroxydisulfuric acid withthe anolyte and catholyte solution temperature near the freezing pointof the anolyte/catholyte solution.

The patents and the U.S. patent application set forth in thespecification are intended to be incorporated by reference herein.

It is apparent that there has been provided in accordance with thisinvention a process and apparatus for etching copper base materialswhich fully satisfy the objects, means, and advantages set forthhereinbefore. While the invention has been described in combination withspecific embodiments thereof, it is evident that many alternatives,modifications, and variations will be apparent to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallwithin the spirit and broad scope of the appended claims.

What is claimed is:
 1. A process for etching copper or a copper basematerial, said process comprising:generating an aqueous etching solutionconsisting essentially of peroxydisulfuric acid in an effective amountup to about 6N., up to about 500 ppm of a halide addition in aneffective amount for decreasing the rate of decomposition of thepersulfate values, and the balance water, said solution beingcharacterized by the absence of chromium or chromate ions, diazines andpurines; introducing said solution into a cleaning tank; and immersingsaid copper or copper base material in said solution in said tank. 2.The process of claim 1 further comprising:diluting said solutionintroduced into said tank so that said peroxydisulfuric acid is presentin amount from about 0.2N. to about 5.5N.
 3. The process of claim 2further comprising:bleeding concentrated peroxydisulfuric acid solutioninto said solution in said tank to maintain the concentration ofperoxydisulfuric acid in the range of about 0.2N. to about 5.5N.
 4. Theprocess of claim 3 wherein said bleeding step maintains saidperoxydisulfuric acid concentration in a range from about 0.5N. to about2N.
 5. The process of claim 1 further comprising:passing said solutionthrough a heat exchanger prior to introducing said solution into saidtank, said passing step retarding the rate of decomposition of saidperoxydisulfuric acid in said solution by maintaining the temperature ofsaid solution in a range from about 0° C. to about 15° C.
 6. The processof claim 3 wherein said passing step maintains the solution temperaturein the range of about 0° C. to about 10° C.
 7. The process of claim 1wherein said generating step comprises generating an aqueous solutionconsisting essentially of about 2N. to about 5N. peroxydisulfuric acid,from about 10 ppm to about 500 ppm of a halide selected from the groupconsisting of hydrocholoric acid and hydrofluoric acid, and the balancewater.
 8. The process of claim 1 wherein said generating step comprisesgenerating a solution consisting essentially of about 3.5N. to about 5N.peroxydisulfuric acid and said halide addition and water.
 9. The processof claim 1 further comprising:said generating step including preparing afresh batch of said etching solution off-line; withdrawing said solutionfrom said tank after the cleaning power of said solution has beendepleted; and replacing said withdrawn solution by said fresh batchproduced off-line.
 10. The process of claim 1 furthercomprising:withdrawing said solution from said tank after the cleaningpower of said solution has been depleted; and introducing said withdrawnsolution into an electrochemical cell for regeneration of said solution.